net: stmmac: use priv->plat->phy_interface directly

Avoid using a local variable for priv->plat->phy_interface as this
may be modified in the .get_interfaces() method added in a future
commit.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1uASLi-0021QX-HG@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>